Seawater desalination system and energy recovery apparatus

ABSTRACT

The present invention relates to a seawater desalination system for desalinating seawater by removing salinity from the seawater and an energy recovery apparatus which is preferably used in the seawater desalination system. The energy recovery apparatus includes a cylindrical chamber (CH) being installed such that a longitudinal direction of the chamber is placed in a vertical direction, a concentrated seawater port (P1) for supplying and discharging the concentrated seawater, a seawater port (P2) for supplying and discharging the seawater, a flow resistor (23) provided at a concentrated seawater port (P1) side in the chamber (CH), and a flow resistor (23) provided at a seawater port (P2) side in the chamber (CH). Each of the flow resistor (23) provided at the concentrated seawater port (P1) side and the seawater port (P2) side comprises at least one perforated circular plate, and each perforated circular plate has a plurality of holes formed in an outer circumferential area outside a circle having a predetermined diameter on the perforated circular plate.

This is a division of U.S. patent application Ser. No. 15/504,230 filedFeb. 15, 2017, which is the national phase of PCT/JP2015/074421 filedAug. 28, 2015, which claims the benefit of Japanese Patent ApplicationNo. 2014-177469 filed Sep. 1, 2014, each which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a seawater desalination system fordesalinating seawater by removing salinity from the seawater and anenergy recovery apparatus which is preferably used in the seawaterdesalination system.

BACKGROUND ART

Conventionally, as a system for desalinating seawater, there has beenknown a seawater desalination system in which seawater passes through areverse-osmosis membrane-separation apparatus to remove salinity fromthe seawater. In the seawater desalination system, the intake seawateris processed to have certain water qualifies by a pretreatment system,and the pretreated seawater is delivered into the reverse-osmosismembrane-separation apparatus under pressure by a high-pressure pump.Part of the high-pressure seawater in the reverse-osmosismembrane-separation apparatus passes through a reverse-osmosis membraneagainst the osmotic pressure and is desalinated, and fresh water(permeate or desalted water) is taken out from the reverse-osmosismembrane-separation apparatus. The remaining seawater is discharged in aconcentrated state of a high salt content as a concentrated seawater(brine) from the reverse-osmosis membrane-separation apparatus. Thelargest operational cost in the seawater desalination system is energycost, and it depends heavily on energy for pressurizing the pretreatedseawater up to such a pressure to overcome the osmotic pressure, i.e. upto the reverse-osmosis pressure. That is, the operational cost of theseawater desalination system is greatly affected by pressurizing energyof the seawater by the high-pressure pump.

Specifically, more than half of the electric expenses as the highestcost in the seawater desalination system are consumed to operate thehigh-pressure pump for pressurizing the seawater. Therefore, pressureenergy possessed by the high-pressure concentrated seawater (reject)with the high salt content which has been discharged from thereverse-osmosis membrane-separation apparatus is utilized forpressurizing part of the seawater. Therefore, as a means for utilizingthe pressure energy of the concentrated seawater discharged from thereverse-osmosis membrane-separation apparatus to pressurize part of theseawater, there has been utilized an energy recovery chamber in which aninterior of a cylinder is separated into two spaces by a piston arrangedto be movable in the cylinder, a concentrated seawater port is providedin one of the two separated spaces to introduce and discharge theconcentrated seawater, and a seawater port is provided in the other ofthe two separated spaces to introduce and discharge the seawater.

FIG. 21 is a schematic view showing a configuration example of aconventional seawater desalination system. As shown in FIG. 21, seawaterpumped into the seawater desalination system by an intake pump (notshown) is processed to have certain water qualities by a pretreatmentsystem for removing suspended matter or the like, and then thepretreated seawater is delivered via a seawater supply line 1 into ahigh-pressure pump 2 that is driven by a motor M. The seawater which hasbeen pressurized by the high-pressure pump 2 is supplied via a dischargeline 3 to a reverse-osmosis membrane-separation apparatus 4 having areverse-osmosis membrane (RO membrane). The reverse-osmosismembrane-separation apparatus 4 separates the seawater into concentratedseawater with a high salt content and fresh water with a low saltcontent and obtains the fresh water from the seawater. At this time, theconcentrated seawater with a high salt content is discharged from thereverse-osmosis membrane-separation apparatus 4, and the, dischargedconcentrated seawater still has a high-pressure. A concentrated seawaterline 5 for discharging the concentrated seawater from thereverse-osmosis membrane-separation apparatus 4 is connected via acontrol valve 6 to a concentrated seawater port P1 of an energy recoverychamber 10. A seawater supply line 1 for supplying the pretreatedseawater having a low pressure is branched at an upstream side of thehigh-pressure pump 2 and is connected via a valve 7 to a seawater portP2 of the energy recovery chamber 10. The energy recovery chamber 10 hasa piston 16 therein, and the piston 16 is arranged to be movable in theenergy recovery chamber 10 while separating the interior of the energyrecovery chamber 10 into two volume chambers.

The seawater pressurized by utilizing a pressure of the concentratedseawater in the energy recovery chamber 10 is supplied via the valve 7to a booster pump 8. The control valve 6, the valve 7 and the energyrecovery chamber 10 constitute an energy recovery apparatus 11. Then,the seawater is further pressurized by the booster pump 8 so that theseawater has the same pressure level as the discharge line 3 of thehigh-pressure pump 2, and the pressurized seawater merges via a valve 9into the discharge line 3 of the high-pressure pump 2 and is thensupplied to the reverse-osmosis membrane separation apparatus 4.

FIG. 22 is a schematic view showing a configuration example of theconventional seawater desalination system comprising the two controlvalves 6, the two energy recovery chambers 10 and the two valves 7 whichare the components of the energy recovery apparatus shown in FIG. 21. Asshown in FIG. 22, since the energy recovery apparatus 11 has the twoenergy recovery chambers 10, 10, the energy recovery apparatus 11 isoperated such that while the concentrated seawater is supplied to one ofthe two energy recovery chambers 10, 10, the concentrated seawater isdischarged from the other of the energy recovery chambers. Therefore,since the high-pressure seawater can be discharged at all times(continuously) from the apparatus by alternating suction of thelow-pressure seawater and discharge of the high-pressure seawater, theflow rate of the seawater supplied to the reverse-osmosis membraneseparation apparatus 4 can be kept constant and the fresh water can beobtained at a constant flow rate from the reverse-osmosis membraneseparation apparatus 4.

In the above-described conventional energy recovery chamber, the pistonin the energy recovery chamber is brought into sliding contact with theinner wall of the chamber, and thus the sliding member of the piston isrequired to be periodically replaced due to wear of the sliding member.Further, the inner diameter of the long chamber is required to bemachined with high accuracy so as to fit with the outer shape of thepiston, and thus machining cost is very expensive.

Therefore, the applicants of the present invention have proposed anenergy recovery chamber having no piston in Japanese laid-open patentpublication No. 2010-284642 by employing the system for pressurizing theseawater directly with the concentrated seawater by introducing theseawater and the high-pressure concentrated seawater discharged from thereverse-osmosis membrane (RO membrane) into a cylindrical and elongatedchamber, which is used as an energy exchange chamber.

FIG. 23 is a cross-sectional view showing an energy recovery chamber 10having no piston. As shown in FIG. 23, the energy recovery chamber 10comprises a long chamber body 11 having a cylindrical shape, and endplates 12 for closing both opening ends of the chamber body 11. Achamber CH is formed in the chamber body 11, and a concentrated seawaterport P1 is formed in one of the end plates 12 and a seawater port P2 isformed in the other of the end plates 12. The concentrated seawater portP1 and the seawater port P2 are disposed on the central axis of thecylindrical chamber body 11. The inner diameter of the chamber CH is setto φD, and the inner diameter of the concentrated seawater port P1 andthe seawater port. P2 is set to φd.

The energy recovery chamber 10 is installed vertically. The chamber CHis disposed vertically in consideration of the effect of a difference inspecific gravity between the concentrate seawater and the seawater, andthe port P1 for the concentrated seawater having large specific gravityis disposed at a lower part of the chamber CH and the port P2 for theseawater having small specific gravity is disposed at an upper part ofthe chamber CH. Specifically, the long chamber body 11 having acylindrical shape is disposed such that a longitudinal direction (axialdirection) of the chamber is placed in a vertical direction. Theconcentrated seawater port P1 is provided at the lower part of thechamber CH so as to supply and discharge the concentrated seawater atthe lower part of the chamber CH, and the seawater port P2 is providedat the upper part of the chamber CH so as to supply and discharge theseawater at the upper part of the chamber CH. The entire length of thechamber CH is L. In the chamber CH, a flow resistor 13 is disposed at aposition spaced by a distance L1 in the axial direction from theconcentrated seawater port P1, and a flow resistor 13 is disposed at aposition spaced by a distance L1 in the axial direction from theseawater port P2. The flow resistor 13 comprises a single perforatedplate.

In the energy recovery chamber 10 shown in FIG. 23, a fluid flows infrom the respective ports P1, P2 having a small diameter, and the fluidflow having a large velocity distribution at a central part of thechamber is dispersed in a diametrical direction of the chamber CH by theflow resistor 13 and is thus regulated to form a uniform flow in thecross-section of the chamber. Therefore, two fluids are pushed andpulled in such a state that the interface between the seawater and theconcentrated seawater is maintained horizontally, and thus the energytransmission is performed while maintaining the state in which theseawater and the concentrated seawater having different saltconcentrations are less likely to be mixed in the chamber.

FIG. 24 is a cross-sectional view showing the energy recovery chamber 10in which two perforated plates spaced by a predetermined distance aredisposed near the respective ports as a flow resistor disposed near eachport in FIG. 23. As shown in FIG. 24, in the chamber CH, a firstperforated plate 14 is provided at a position spaced by a distance L1 inthe axial direction from the concentrated seawater port P1, and a secondperforated plate 15 is provided at a position spaced by a distance L2 inthe axial direction from the first perforated plate 14. Similarly, afirst perforated plate 14 is provided at a position spaced by a distanceL1 in the axial direction from the seawater port P2, and a secondperforated plate 15 is provided at a position spaced by a distance L2 inthe axial direction from the first perforated plate 14. The twoperforated plates 14 and 15 constitute a flow resistor 13.

Other structural elements of the energy recovery chamber 10 shown inFIG. 24 are the same as those in the energy recovery chamber 10 shown inFIG. 23.

The applicant of the present invention has found that in the aboveenergy recovery apparatus, when the fluid which flows into the chamberhas a high flow velocity, or depending on dimension and shape of theperforated plate or the arrangement position of the perforated plate,i.e., the distance L1 in FIG. 23 or the distances L1, L2 in FIG. 24, theeffect of dispersion and regulation of the fluid is not sufficient andnon-uniform flow having a high flow velocity still at the central partof the chamber is formed. Thus, the applicant of the present inventionhas proposed its solution in Japanese patent application No. 2013-078012(has not been disclosed). Specifically, as shown in FIG. 25, in thechamber CH, a flow resistor 23 is disposed at a position spaced by adistance L1 in the axial direction from the concentrated seawater portP1, and a flow resistor 23 is disposed at a position spaced by adistance L1 in the axial direction from the seawater port P2. As shownin FIG. 26 which is a plan view of the flow resistor, the flow resistor23 comprises a single perforated plate which has a circular plate shapehaving an outer diameter (φD) equal to the inner diameter of the chamberand has a plurality of small holes 23 h having a diameter φdk1 formedoutside a hypothetical circle (φdc) at a central part of the circularplate and no hole inside the hypothetical circle (center side).Specifically, the perforated plate which has a blocked central portionis disposed.

FIG. 27 is a view showing the flow distribution by Computational Fluid.Dynamics in the vicinity of the seawater port in the case where the flowresistor 23 comprising a perforated plate whose central portion isblocked is installed horizontally as shown in FIG. 25. Arrows in FIG. 27are as follows: Flow velocity of fluid is shown by a length of arrow andflow direction of fluid is shown by a direction of arrow.

Because the fluid flows into the chamber CH from the seawater port P2having a small diameter, the fluid near the port of the chamber has avelocity distribution having a large stream at the central part of thechamber. The high-velocity flow of fluid at the central part collideswith the blockage portion of the perforated plate facing the port, andthen the flow of fluid is directed horizontally along the plate towardthe outer circumference of the chamber. The fluid passes through theperforated plate only from the small holes formed at the outercircumferential portion of the perforated plate and flows downstream,and part of the horizontal flow of fluid is directed upwardly along theside surface of the chamber, thus generating large vortices at the outercircumferential portion of the chamber. At this time, the flow of fluidcollides with the blockage portion of the perforated plate and is thendirected toward the outer circumference of the chamber, and thehigh-velocity fluid which flows into the chamber from the port is sloweddown. The flow of fluid which has passed through the small holes at theouter circumferential portion of the perforated plate is directedtowards the outer circumferential side once at its central part, andthen gathers in the central part of the chamber again. Although thevortices generate at the downstream side of the blockage portion of theperforated plate, the velocity of flow and the direction of flow can beuniformized in the A-A cross-section spaced by a predetermined distancefrom the perforated plate shown in FIG. 27 to the center of the chamber.

CITATION LIST Patent Literature

Patent document 1: Japanese laid-open patent publication No. 2010-284642

SUMMARY OF INVENTION Technical Problem

The applicant of the present invention has made an analysis of thecharacteristics of flows in the chamber by utilizing Computational FluidDynamics (CFD) focusing on a velocity distribution of fluid which flowsin from the port of the energy recovery apparatus in FIGS. 25 and 26. Asa result, the applicant of the present invention has found that theperforated plate having the circular blockage portion at the centralpart of the circular plate has velocity dependency, and the velocitiesand directions of the flows in the chamber become uniform by theperforated plate in a limited range of inflow velocity, and that whenthe flow velocity of the fluid which flows in from the port having asmall diameter varies, the velocity distribution at an evaluationsection that is spaced by a predetermined distance from the perforatedplate toward the center of the chamber also varies, thus lowering theflow uniformity.

FIG. 28 is a view showing the flow distribution by Computational FluidDynamics (CFD) in the vicinity of the seawater port, where a perforatedplate having a circular blockage portion at a central part of a circularplate is disposed in the chamber and the flow velocity of fluid is setto be three times greater than the case of FIG. 27.

As a result of the Computational Fluid Dynamics (CFD) and analysis, ithas been found that the variation in the velocity distribution dependingon the flow velocity of fluid is caused by the size of a vortexgenerated at the back side (downstream side) of the circular blockageportion at the central part of the circular plate.

A comparison between FIG. 27 and FIG. 28 shows that the directions offlows G passing through the vicinity of the outer edge of the centralblockage portion are more inclined toward the outer peripheral side ofthe chamber in FIG. 28 than those in FIG. 27, the flow velocity of fluidbeing higher in FIG. 28 than in FIG. 27. The vortex (denoted by Vx inFIG. 28) generated at the back side of the central blockage portionspreads in a laterally longer pattern. Thus, it is analyzed that theaction by which the flows directed once from the center toward the outerperipheral side are strongly attracted to the center is produced in thedownstream side of the vortex, resulting in a lowering of the flowuniformity at the evaluation section that is spaced by a predetermineddistance from the perforated plate when the flow velocity of fluid fromthe port becomes high.

As described above, the perforated plate or mesh having the circularblockage portion at its central part has the action to disperse the flowin the chamber uniformly at all times if the flow velocity of fluid fromthe port having a small diameter falls within a predetermined range.However, the desired action may not be obtained if the flow velocity offluid from the port is greatly changed. In particular, when the fluid issupplied to and discharged from the chamber from an actual port, thefluid does not flow at a constant velocity from the port into thechamber at all times, but the flow velocity greatly varies in one cycle.Specifically, the increase in the flow velocity limit for obtaining adesired flow uniformizing action allows the energy recovery chamber tobe applicable to a wide range of flow rate to be processed. The flowvelocity limit for the perforated plate having the circular blockageportion at its central part has been about 250 mm/s. However, even if aflow velocity exceeds this flow velocity limit, the flow uniformizingaction does not completely disappear, but the flow velocity distributionbecomes greater than a predetermined threshold value.

Here, a uniform flow of fluid means that speed and directions of fluidflow are uniform in a certain horizontal cross-section of the chamber.Specifically, the case where flow speed of fluid (scalar) and flowdirection of fluid (vector) in a certain horizontal cross-section of thechamber are identically distributed at any position in the horizontalcross-section is defined as a completely uniform flow of fluid.Specifically, as shown in FIG. 29, flows at arbitrary points Pn, Pm inthe horizontal cross-section are shown by arrows representing flowmagnitudes which are Vn, Vm, respectively. In this case, when angles (α,β) between the arrows and the auxiliary lines X, Y (X is perpendicularY) on the horizontal cross-section are the same (α_(n)=α_(m),β_(n)=β_(m)), the flows at the points Pn, Pm are defined as a uniformflow. When the angles α, β are the same at any position in thehorizontal cross-section, such flow is defined as a completely uniformflow of fluid. Here, the condition of being closer to this state isdefined as a uniform flow. Because a cylindrical chamber wall exists atthe outer circumference in the horizontal cross-section as a verticalwall surface, as both the angles α, β become closer to a right angle,more uniform flow is formed.

When the fluid flows into the chamber CH from the respective ports P1,P2 having a small diameter, the fluid flows through the central part ata high velocity and through the outer circumferential part at a lowvelocity in the horizontal cross-section of the chamber in the vicinityof the respective ports P1, P2. Here, to make the dispersion of velocitydistribution in the horizontal cross-section small by averaging thefluid flow so that the fluid flows through the central part at a lowvelocity and through the outer circumferential part at a high velocityis defined as “an uniformizing action” “for uniformizing the flow”.Further, “regulating the flow” means that distribution of flow velocityis changed, and to form a uniform flow as a result of changing thedistribution of flow velocity by regulating the flow is defined as“uniformization of flow by regulating the flow”.

The pushing and pulling of the seawater and the concentrated seawatermeans the operation for pushing out (pushing) the seawater from thechamber while pressurizing the seawater with the concentrated seawater,and then drawing in and discharging (pulling) the concentrated seawaterwith the seawater by switching the valve 6. In FIGS. 24 and 25, aboundary portion of the two fluid where the seawater and theconcentrated seawater are brought into contact with each other is formedin the chamber space having a length La between the flow resistors 13,13. The boundary portion reciprocates in La by pushing and pulling ofthe seawater and the concentrated seawater, and thus the seawater andthe concentrated seawater are controlled so that the seawater is notdischarged from the concentrated seawater port P1 and the concentratedseawater is not discharged from the seawater port P2. In the case wherethe chamber is installed vertically, i.e., is configured such that theconcentrated seawater is located at the lower part of the chamber andthe seawater is located at the upper part of the chamber, the pushingand pulling of the seawater and the concentrated seawater have the samemeaning as the pushing up the seawater and pushing down the concentratedseawater.

The mixing of the seawater and the concentrated seawater at the boundaryportion is accelerated by pushing and pulling of the seawater and theconcentrated seawater. However, by allowing the flow of the seawater andthe concentrated seawater above and below the boundary portion to be auniform flow in the zone of La in the chamber, the phenomenon in whichthe boundary surface causes turbulence flow diffusion by non-uniformityof the flow to mix the seawater and the concentrated seawater issuppressed. At the same time, by maintaining the boundary portionhorizontally, the pushing and pulling of the seawater and theconcentrated seawater can be performed as if there is a hypotheticalpiston.

When pushing and pulling of the seawater and the concentrated seawaterare performed in a state of non-uniform flow in the cross-section of thechamber, mixing of the seawater and the concentrated seawater caused byturbulent flow diffusion in the chamber progresses, and thus theseawater having a high salt content is discharged from the energyrecovery apparatus. As a result, the salt content of the seawatersupplied to the reverse-osmosis membrane-separation apparatus increases,thus decreasing the amount of fresh water obtained from thereverse-osmosis membrane-separation apparatus, or the pressure of theseawater supplied to the reverse-osmosis membrane-separation apparatusfor obtaining the same amount of fresh water increases, thus increasingthe energy per unit amount of produced fresh water.

The present invention has been made in view of the above drawbacks. Itis therefore an object of the present invention to provide an energyrecovery apparatus, having flow resistors disposed respectively at theconcentrated seawater port side and the seawater port side of thechamber, which can perform pressure transmission from the high-pressureconcentrated seawater to the seawater while suppressing mixing of thetwo fluids at the boundary portion where the two fluids are brought intocontact with each other by the effect for regulating the flow of fluidby the flow resistors to uniformize the flow of fluid even if thehigh-velocity flow of fluid collides with the central part of the flowresistor corresponding to the port diameter, and can prevent thedischarge of the seawater having a high salt content which may occur bymixing of the seawater and the concentrated seawater in the energyrecovery apparatus.

In particular, the object of the present invention is to provide anenergy recovery apparatus having a configuration which can exert aneffect for forming a uniform flow in a wide range of flow velocity andhave less dependency on the flow velocity of fluid which flows in fromthe port, as the effect for forming a uniform flow by regulating theflow of fluid with the flow resistors.

Solution to Problem

In order to achieve the above object, according to a first aspect of thepresent invention, there is provided an energy recovery apparatus forconverting pressure energy of concentrated seawater discharged from areverse-osmosis membrane-separation apparatus to pressure energy ofseawater in a seawater desalination system for producing fresh waterfrom the seawater by supplying the seawater pressurized by a pump to thereverse-osmosis membrane-separation apparatus to separate the seawaterinto fresh water and concentrated seawater, the energy recoveryapparatus comprising: a cylindrical chamber having a space forcontaining concentrated seawater and seawater therein, the chamber beinginstalled such that a longitudinal direction of the chamber is placed ina vertical direction; a concentrated seawater port provided at a lowerpart of the chamber for supplying and discharging the concentratedseawater; a seawater port provided at an upper part of the chamber forsupplying and discharging the seawater; a flow resistor provided at aconcentrated seawater port side in the chamber; and a flow resistorprovided at a seawater port side in the chamber; wherein each of theflow resistor provided at the concentrated seawater port side and theseawater port side comprises at least one perforated circular plate; andwherein each perforated circular plate has a plurality of holes formedin an outer circumferential area outside a hypothetical circle which isconcentric with the perforated circular plate and has a predetermineddiameter, the holes being formed so that an aperture ratio is graduallyincreased from an outer diameter of the hypothetical circle having thepredetermined diameter toward an outer diameter of the perforatedcircular plate.

According to the present invention, the concentrated seawater issupplied to and discharged from the chamber through the concentratedseawater port provided at the lower part of the chamber, and theseawater is supplied to and discharged from the chamber through theseawater port provided at the upper part of the chamber. According tothe present invention, the high-velocity fluid which has flowed into thechamber collides with the area having no hole located at the centralportion of the perforated circular plate which has holes at an outercircumferential area outside a predetermined diameter, and is thusregulated so that the flow of fluid is dispersed in a radial directionof the chamber and slowed down, and then the fluid flows downstream fromthe area having holes at the outer circumferential area of theperforated circular plate. Therefore, the fluid which flows into thechamber and has a large stream at the central part of the chamberreduces its speed and is dispersed, and thus more uniform flow velocitydistribution in the cross-section of the chamber can be formed. Theconcentrated seawater and the seawater which are regulated by theperforated circular plates form a boundary portion by a difference inspecific gravity, and the concentrated seawater at the lower side pushesup the seawater and the seawater at the upper side pushes down theconcentrated seawater by pushing and pulling. Thus, while theconcentrated seawater and the seawater are separated one above the otherand mixing of the concentrated seawater and the seawater at the boundaryportion where the two fluids are brought into contact with each other issuppressed, the pressure can be transmitted from the high-pressureconcentrated seawater to the seawater.

Further, because the holes are formed in the perforated circular plateso that an aperture ratio is gradually increased from an outer diameterof a hypothetical circle having a predetermined diameter toward an outerdiameter of the perforated circular plate, even if the flow velocity offluid which flows in from the port is changed to a wide range of flowvelocity, the flow of fluid at the back side of the perforated circularplate is less likely to be changed, and thus a uniform flow can beformed.

According to a preferred aspect of the present invention, an area of theperforated circular plate that is free of the holes is an area of a starpolygon which has a circle having a predetermined diameter as anincircle and a circle having a diameter equal to or smaller than theouter diameter of the perforated circular plate and greater than thediameter of the hypothetical circle as a circumcircle.

According to the present invention, by providing strength and weaknessin the blockage portion, i.e. variation in an aperture ratio in acircumferential direction of the perforated plate, the flow of fluid ina radial direction at the downstream side after the fluid passes throughthe perforated plate is changed by the hole distribution, therebyregulating the flow of fluid so as to be uniformized in a longitudinaldirection of the entire chamber. Further, this configuration has a highuniformizing action in a wide range of flow velocity even if the inflowvelocity from the seawater port and the concentrated seawater port ischanged, and thus has an action for uniformizing the high-velocity flowof fluid whose flow velocity is higher than the conventional flowvelocity.

According to a preferred aspect of the present invention, the perforatedcircular plate serves as a first perforated plate, and a secondperforated plate is provided to be spaced by a predetermined distancefrom the first perforated plate.

According to the present invention, the flow resistor can have a higheruniformizing effect because the flow of fluid which has been dispersedand regulated uniformly by the first perforated plate is furtheruniformized by the second perforated plate disposed at the downstreamside of the first perforated plate.

According to a preferred aspect of the present invention, the energyrecovery apparatus further comprises a doughnut-shaped circular platehaving an opening at a center thereof provided between one of theconcentrated seawater port and the seawater port or both of theconcentrated seawater port and the seawater port, and the flow resistor.

According to the present invention, when the concentrated seawater issupplied to and discharged from the chamber through the concentratedseawater port provided at the lower part of the chamber, and theseawater is supplied to and discharged from the chamber through theseawater port provided at the upper part of the chamber, even if theseawater port and the concentrated seawater port are not located at thechamber axis, the fluid which has flowed in the chamber flows throughthe hole at the central portion of the circular plate toward the centralportion of the flow resistor. Therefore, the flow of fluid isdistributed over the entirety of the chamber from the central portion atthe upstream side of the flow resistor without the deviated flow, andthus the flow of fluid at the downstream side of the flow resistor canbe regulated more uniformly. The concentrated seawater and the seawaterWhich are regulated by the flow resistors form a boundary portion by adifference in specific gravity, and the concentrated seawater at thelower side pushes up the seawater and the seawater at the upper sidepushes down the concentrated seawater by pushing and pulling. Thus,while the concentrated seawater and the seawater are separated one abovethe other and mixing of the concentrated seawater and the seawater atthe boundary portion where the two fluids are brought into contact witheach other is suppressed, the pressure can be transmitted from thehigh-pressure concentrated seawater to the seawater.

According to a second aspect of the present invention, there is providedan energy recovery apparatus for converting pressure energy ofconcentrated seawater discharged from a reverse-osmosismembrane-separation apparatus to pressure energy of seawater in aseawater desalination system for producing fresh water from the seawaterby supplying the seawater pressurized by a pump to the reverse-osmosismembrane-separation apparatus to separate the seawater into fresh waterand concentrated seawater, the energy recovery apparatus comprising: acylindrical chamber having a space for containing concentrated seawaterand seawater therein, the chamber being installed such that alongitudinal direction of the chamber is placed in a vertical direction;a concentrated seawater port provided at a lower part of the chamber forsupplying and discharging the concentrated seawater; a seawater portprovided at an upper part of the chamber for supplying and dischargingthe seawater; a flow resistor provided at a concentrated seawater portside in the chamber; and a flow resistor provided at a seawater portside in the chamber; wherein each of the flow resistor provided at theconcentrated seawater port side and the seawater port side comprises atleast one perforated circular plate; wherein each perforated circularplate has a plurality of holes formed in an outer area outside ahypothetical circle having a predetermined diameter on the perforatedcircular plate, the outer area including a forming area where the holesare densely formed and a non-forming area where no hole is formed; andwherein a bundle-like collected jet flow comprising a group of jet flowswhich pass through the holes in the forming area is defined and astationary fluid formed by blocking the flow passing through theperforated circular plate by the non-forming area is defined, theforming area and the non-forming area being alternately distributed in acircumferential direction of the outer area.

According to a preferred aspect of the present invention, shear takesplace between the collected jet flow and the stationary fluid.

According to a third aspect of the present invention, there is providedan energy recovery apparatus for converting pressure energy ofconcentrated seawater discharged from a reverse-osmosismembrane-separation apparatus to pressure energy of seawater in aseawater desalination system for producing fresh water from the seawaterby supplying the seawater pressurized by a pump to the reverse-osmosismembrane-separation apparatus to separate the seawater into fresh waterand concentrated seawater, the energy recovery apparatus comprising: acylindrical chamber having a space for containing concentrated seawaterand seawater therein, the chamber being installed such that alongitudinal direction of the chamber is placed in a vertical direction;a concentrated seawater port provided at a lower part of the chamber forsupplying and discharging the concentrated seawater; a seawater portprovided at an upper part of the chamber for supplying and dischargingthe seawater; a flow resistor provided at a concentrated seawater portside in the chamber; and a flow resistor provided at a seawater portside in the chamber; wherein each of the flow resistor provided at theconcentrated seawater port side and the seawater port side comprises atleast one perforated circular plate; wherein each perforated circularplate has a plurality of holes formed in an outer area outside ahypothetical circle having a predetermined diameter on the perforatedcircular plate, the outer area including a forming area where the holesare formed and a non-forming area where no hole is formed; and whereinthe non-forming area is joined to the hypothetical circle and forms apetal-shape non-forming area radially extending toward the outercircumference of the perforated circular plate.

According to a fourth aspect of the present invention, there is providedan energy recovery apparatus for converting pressure energy ofconcentrated seawater discharged from a reverse-osmosismembrane-separation apparatus to pressure energy of seawater in aseawater desalination system for producing fresh water from the seawaterby supplying the seawater pressurized by a pump to the reverse-osmosismembrane-separation apparatus to separate the seawater into fresh waterand concentrated seawater, the energy recovery apparatus comprising: acylindrical chamber having a space for containing concentrated seawaterand seawater therein, the chamber being installed such that alongitudinal direction of the chamber is place in a vertical direction;a concentrated seawater port provided at a lower part of the chamber forsupplying and discharging the concentrated seawater; a seawater portprovided at an upper part of the chamber for supplying and dischargingthe seawater; a flow resistor provided at a concentrated seawater portside in the chamber; and a flow resistor provided at a seawater portside in the chamber; wherein each of the flow resistor provided at theconcentrated seawater port side and the seawater port side comprises atleast one perforated circular plate; and wherein each perforatedcircular plate has a plurality of holes formed in an outer area outsidea hypothetical circle having a predetermined radius from the center ofthe perforated circular plate, a plurality of areas where no hole isformed are provided in a circumferential direction of the outer area,and each of the plurality of areas spreads toward an outer diameter ofthe perforated circular plate in a substantially triangular shape havinga bottom side on an arc formed by the hypothetical circle.

According to the present invention, there is provided a seawaterdesalination system for producing fresh water from seawater by supplyingthe seawater pressurized by a pump to a reverse-osmosismembrane-separation apparatus to separate the seawater into fresh waterand concentrated seawater, the seawater desalination system comprising:the above energy recovery apparatus for converting pressure energy ofthe concentrated seawater discharged from the reverse-osmosismembrane-separation apparatus to pressure energy of the seawater.

According to the present invention, the pressure energy of thehigh-pressure concentrated seawater discharged from the reverse-osmosismembrane-separation apparatus can be directly transmitted to theseawater, and mixing of the two fluids when the concentrated seawaterand the seawater are pushed and pulled can be suppressed. Therefore, theseawater having a high salt content is not discharged from the energyrecovery apparatus, and thus the system can be operated without raisingsupply pressure of the seawater to the reverse-osmosismembrane-separation apparatus. Accordingly, the electric power requiredfor operating the system can be reduced.

Advantageous Effects of Invention

According to the present invention, the following effects can beachieved.

1) The high-velocity fluid which has flowed into the chamber isdispersed in a radial direction of the chamber and is slowed down at thearea having no hole at the central part of the perforated circular platewhich has holes at an outer circumferential area outside a predetermineddiameter, and then the fluid flows downstream from the area having theholes at the outer circumferential area of the perforated circularplate. Further, by providing strength and weakness in the blockageportion, i.e. variation in an aperture ratio in a circumferentialdirection of the perforated plate, the flow of fluid in a radialdirection at the downstream side after the fluid passes through theperforated plate is changed by the hole distribution. Therefore, theaction for forming more uniform flow in the cross-section of the chamberis remarkably improved by decelerating and dispersing the fluid whichflows into the chamber and has a large stream. By the uniformizingaction for regulating the flow of fluid by the flow resistor comprisingthe perforated circular plate, while mixing of the concentrated seawaterand the seawater at the boundary portion where the two fluids arebrought into contact with each other is suppressed, the pressure can betransmitted from the high-pressure concentrated seawater to theseawater. Further, because the holes are formed in the perforatedcircular plate so that an aperture ratio is gradually increased from anouter diameter of a hypothetical circle having a predetermined diametertoward an outer diameter of the perforated circular plate, even if theflow velocity of fluid which flows in from the port is changed to a widerange of flow velocity, the flow of fluid at the back side of theperforated circular plate is less likely to be changed, and thus auniform flow can be formed.

2) Because mixing of the concentrated seawater and the seawater in thechamber due to turbulent flow diffusion can be suppressed and theseawater having a high salt content is not delivered to thereverse-osmosis membrane-separation apparatus, the reverse-osmosismembrane-separation apparatus can provide its sufficient performance andthe replacement cycle of the reverse-osmosis membrane itself can beprolonged.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a configuration example of a seawaterdesalination system according to the present invention;

FIG. 2 is a schematic cross-sectional view showing an energy recoverychamber of the present invention which is applied to the seawaterdesalination system shown in FIG. 1;

FIG. 3 is a plan view showing an example of the flow resistor;

FIG. 4 is a view showing an example of another flow resistor, and a planview showing the flow resistor comprising a single mesh plate;

FIG. 5 is an enlarged plan view of holes of the perforated plate in FIG.3;

FIG. 6 is an enlarged plan view of a wire material and opening portionsof a mesh plate in FIG. 4;

FIGS. 7(a), 7(b), and 7(c) are graphs showing average aperture ratios atdiametrical positions of three types of flow resistors;

FIGS. 8(a), 8(b) are views showing the flow distribution of CFD(Computational Fluid Dynamics) in the vicinity of the seawater port inthe case where the flow resistor comprising a perforated plate whosecentral portion is blocked in a star hexagonal shape is installedhorizontally as shown in FIG. 2;

FIGS. 9(a), 9(b) are views showing the flow distribution of CFD(Computational Fluid Dynamics) in another cross-section in the vicinityof the seawater port in the case where the flow resistor comprising aperforated plate whose central portion is blocked in a star hexagonalshape is installed horizontally as shown in FIG. 2;

FIGS. 10(a), 10(b) are views showing the flow distribution of CFD(Computational Fluid Dynamics) in still another cross-section in thevicinity of the seawater port in the case where the flow resistorcomprising a perforated plate whose central portion is blocked in a starhexagonal shape is installed horizontally as shown in FIG. 2;

FIG. 11(a) is a schematic cross-sectional view showing an energyrecovery chamber of an energy recovery apparatus according to anotherembodiment of the present invention,

FIG. 11(b) is a plan view showing the respective perforated platesdisposed in the energy recovery chamber shown in FIG. 11(a);

12 is an enlarged plan view of the second perforated plate shown in FIG.11(b);

FIGS. 13(a) and 13(b) are views showing the flow distribution of theinterior of the chamber in the case where seawater flows into thechamber from the seawater port located in the vicinity of the upper partof the chamber in FIG. 11(a);

FIGS. 14(a) and 14(b) are views showing the flow distribution in anothercross-section of the interior of the chamber in the case where seawaterflows into the chamber from the seawater port located in the vicinity ofthe upper part of the chamber in FIG. 11(a);

FIGS. 15(a) and 15(b) are views showing the flow distribution in stillanother cross-section of the interior of the chamber in the case whereseawater flows into the chamber from the seawater port located in thevicinity of the upper part of the chamber in FIG. 11(a);

FIG. 16 is a plan view showing a perforated plate of the flow resistoraccording to another embodiment;

FIG. 17 is an enlarged plan view of holes formed in the perforated plateshown in FIG. 16;

FIG. 18 is a cross-sectional view showing a chamber of the energyrecovery apparatus according to still another embodiment of the presentinvention;

FIG. 19 is a plan view of the holed circular plate;

FIG. 20 is a schematic cross-sectional view showing an energy recoverychamber of the energy recovery apparatus according to another embodimentof the present invention;

FIG. 21 is a schematic view showing a configuration example of aconventional seawater desalination system;

FIG. 22 is a schematic view showing a configuration example of theconventional seawater desalination system comprising the two controlvalves, the two energy recovery chambers and the two valves which arethe components of the energy recovery apparatus shown in FIG. 21;

FIG. 23 is a cross-sectional view showing a conventional energy recoverychamber having no piston;

FIG. 24 is a cross-sectional view showing the energy recovery chamber inwhich two perforated plates spaced by a predetermined distance aredisposed near the respective ports as a flow resistor in FIG. 23;

FIG. 25 is a cross-sectional view showing an energy recovery chamberwhich has flow resistors having a circular blockage portion at itscenter, which has been proposed in Japanese patent application No.2013-078012;

FIG. 26 is a plan view showing a perforated plate which has a circularblockage portion, which has been proposed in Japanese patent applicationNo. 2013-078012;

FIG. 27 is a view showing the flow distribution by CFD (ComputationalFluid Dynamics) of the interior of the chamber in the case wherehigh-velocity seawater flows into the chamber from the seawater portlocated in the vicinity of the upper part of the chamber in FIG. 25;

FIG. 28 is a view showing the flow distribution by CFD (ComputationalFluid Dynamics) of the interior of the chamber in the case wherehigher-velocity seawater flows into the chamber from the seawater portlocated in the vicinity of the upper part of the chamber in FIG. 25; and

FIG. 29 is a view showing the uniformity of flows at any points Pn, Pmin the horizontal cross-section in the chamber.

DESCRIPTION OF EMBODIMENTS

An energy recover apparatus according to preferred embodiments of thepresent invention will be described below with reference to FIGS. 1through 20. Identical or corresponding parts are denoted by identicalreference numerals in FIGS. 1 through 20 and will not be described induplication.

FIG. 1 is a schematic view showing a configuration example of a seawaterdesalination system according to the present invention. As shown in FIG.1, seawater pumped into the seawater desalination system by an intakepump (not shown) is processed to have certain water qualities by apretreatment system, and then the pretreated seawater is delivered via aseawater supply line 1 into a high-pressure pump 2 that is driven by amotor M. The seawater which has been pressurized by the high-pressurepump 2 is supplied via a discharge line 3 to a reverse-osmosismembrane-separation apparatus 4 having a reverse-osmosis membrane (ROmembrane). The reverse-osmosis membrane-separation apparatus 4 separatesthe seawater into concentrated seawater with a high salt content andfresh water with a low salt content and obtains the fresh water from theseawater. At this time, the concentrated seawater with a high saltcontent is discharged from the reverse-osmosis membrane-separationapparatus 4, and the discharged concentrated seawater still has ahigh-pressure. A concentrated seawater line 5 for discharging theconcentrated seawater from the reverse-osmosis membrane-separationapparatus 4 is connected via a control valve 6 to a concentratedseawater port P1 of an energy recovery chamber 20. A seawater supplyline 1 for supplying the pretreated seawater having a low pressure isbranched at an upstream side of the high-pressure pump 2 and isconnected via a valve 7 to a seawater port P2 of the energy recoverychamber 20. The energy recovery chamber 20 performs energy transmissionfrom the concentrated seawater to the seawater while separating twofluids by a boundary region between the concentrated seawater and theseawater.

The seawater pressurized by utilizing a pressure of the concentratedseawater in the energy recovery chamber 20 is supplied via a valve 7 toa booster pump 8. Then, the seawater is further pressurized by thebooster pump 8 so that the seawater has the same pressure level as thedischarge line 3 of the high-pressure pump 2, and the pressurizedseawater merges via a valve 9 into the discharge line 3 of thehigh-pressure pump 2 and is then supplied to the reverse-osmosismembrane-separation apparatus 4. On the other hand, the concentratedseawater which has pressurized the seawater and lost the energy isdischarged front the energy recovery chamber 20 via the control valve 6to a concentrated seawater discharge line 17.

When the pressure of the discharge line 3 of the high-pressure pump 2 is6.5 MPa for example, the pressure is slightly lowered by pressure lossof the RO membrane module of the reverse-osmosis membrane-separationapparatus 4, and the concentrated seawater having a pressure of 6.4 MPais discharged from the reverse-osmosis membrane-separation apparatus 4.When the pressure energy of the concentrated seawater acts on theseawater, the seawater is pressurized to the same pressure (6.4 MPa) butthe pressure is decreased by pressure loss of the energy recoveryapparatus itself when the seawater flows through the energy recoveryapparatus, and the seawater having a pressure of 6.3 MPa far example isdischarged from the energy recovery apparatus. The booster pump 8slightly pressurizes the seawater from 6.3 MPa to 6.5 MPa, and theseawater merges into the discharge line 3 of the high-pressure pump 2and is supplied to the reverse-osmosis membrane-separation apparatus 4.The booster pump 8 only needs to pressurize the seawater to make up forsuch a small pressure loss, and thus a small amount of energy isconsumed in the booster pump 8.

It is assumed that 100% of an amount of seawater is supplied to thereverse-osmosis membrane-separation apparatus 4, 40% of the amount ofthe seawater can be changed to fresh water. The remaining 60% of theamount of the seawater is concentrated and discharged from thereverse-osmosis membrane-separation apparatus 4 as concentratedseawater. Then, the pressure of the 60% concentrated seawater istransmitted and recovered by the seawater in the energy recoveryapparatus, and the seawater haying an increased pressure is dischargedfrom the energy recovery apparatus. Therefore, the seawater having ahigh pressure equivalent to the seawater pressurized by thehigh-pressure pump can be obtained, with a small amount of energyconsumed by the booster pump. Thus, the energy which is consumed by thehigh-pressure pump to produce the fresh water can be about half of theenergy in the case of no energy recovery apparatus.

FIG. 2 is a schematic cross-sectional view showing the energy recoverychamber of the present invention which is applied to the seawaterdesalination system shown in FIG. 1. As shown in FIG. 2, the energyrecovery chamber 20 comprises a long chamber body 21 having acylindrical shape, and end plates 22 for closing both opening ends ofthe chamber body 21. A chamber CH is formed in the chamber body 21, anda concentrated seawater port P1 is formed in one of the end plates 22and a seawater port P2 is formed in the other of the end plates 22.

The energy recovery chamber 20 is installed vertically. The chamber CHis disposed vertically in consideration of the effect of a difference inspecific gravity between the concentrate seawater and the seawater, andthe port P1 for the concentrated seawater having large specific gravityis disposed at a lower part of the chamber CH and the port P2 for theseawater having small specific gravity is disposed at an upper part ofthe chamber CH. That is, the long chamber body 21 having a cylindricalshape is installed such that a longitudinal direction (axial direction)of the chamber is placed in a vertical direction. The concentratedseawater port P1 is provided at the lower part of the chamber CH forsupplying and discharging the concentrated seawater at the lower part ofthe chamber CH, and the seawater port P2 is provided at the upper partof the chamber CH for supplying and discharging the seawater at theupper part of the chamber CH. The entire length of the chamber CH is L.In the chamber CH, a flow resistor 23 is disposed at a position spacedby a distance L1 in the axial direction from the concentrated seawaterport P1, and a flow resistor 23 is disposed at a position spaced by adistance L1 in the axial direction from the seawater port P2.

FIG. 3 is a plan view showing an example of the flow resistor. As shownin FIG. 3, the flow resistor 23 has a circular plate shape having anouter diameter (φD) equal to the inner diameter of the chamber. The flowresistor 23 comprises a single perforated plate which has a plurality ofsmall boles 23 h having a diameter φdk1 formed outside a hypotheticalpolygon (particularly, a concave polygon, a star hexagon (six-pointedstar, hexagram), and the like) and has no hole inside the hypotheticalpolygon (central side). The hypothetical polygon has a centralhypothetical circle (diameter: φdc) as an incircle and an outerhypothetical circle (diameter: φdr) as a circumcircle. Specifically, theflow resistor 23 comprises a perforated plate which is blocked in itscentral portion and in part of its outer circumferential portion. InFIG. 3, the points of intersection between the incircle as ahypothetical circle and the hypothetical polygon are denoted by Pdc, andthe points of intersection between the circumcircle as a hypotheticalcircle and the hypothetical polygon are denoted by Pdr.

The diameter (φdc) of the central hypothetical circle on the perforatedplate is equal to or slightly greater than the inner diameter φds of theseawater port and the inner diameter φdb of the concentrated seawaterport, so that the high-velocity flow of fluid flowing in from each ofthe ports collides with the blockage portion and is slowed down.However, if the blockage portion is excessively larger than each of theports, the flows of fluid passing through a plurality of small holes 23h provided at an outer circumferential side are localized in an outercircumferential region, thus making the flow uniformizing action smalladversely. Therefore, the diameter of the central hypothetical circle issubstantially the same as the inner diameter of each of the ports.

The central hypothetical circle of the flow resistor 23 is positioned soas to be concentric with the outer circumference of the circular plateof the flow resistor 23. As shown in FIG. 2, the seawater port and theconcentrated seawater port are disposed on the axis of the cylindricalchamber so that the high-velocity flow of fluid flowing into the chamberfrom each of the ports collides with the blockage portion that isdefined by the central hypothetical circle.

The diameter (φdr) of the hypothetical circle that circumscribes thestar hexagon is smaller than the outer diameter (φD) of the flowresistor 23.

The flow resistor 23 comprising the perforated plate which is blocked bythe star hexagon, has a function to regulate the flow of fluid at thedownstream side of the flow resistor 23 so as to be uniformized in theentire chamber by imparting an appropriate flow resistance to the flowof fluid at the upstream side of the flow resistor 23 in the chamber CH.

The outer edges that interconnect adjacent corners of the polygon maynot necessarily be straight sides.

FIG. 4 is a view showing an example of another flow resistor, and a planview showing a flow resistor which comprises a single mesh plate. Asshown in FIG. 4, the flow resistor 23 comprises a mesh material which isformed into a circular plate shape having an outer diameter φD byweaving a wire material. Another plate 30, which is of a star hexagonhaving a hypothetical circle (diameter: φdc) at the central part as anincircle and a hypothetical circle (diameter: φdr) at the outercircumference as a circumcircle, is attached to the circular platecomprising the mesh material. A fluid flows through a portion, where themesh material is exposed, outside the star hexagon, but does not flowthrough the plate 30 having the star hexagonal shape.

The flow resistor 23 having a configuration which is blocked by the starhexagon, has a function to regulate the flow of fluid at the downstreamside of the flow resistor 23 so as to be uniformized in the entirechamber by imparting an appropriate flow resistance to the flow of fluidat the upstream side of the flow resistor 23 in the chamber CH. Theperforated plate shown in FIG. 3 and the mesh plate shown in FIG. 4 arecollectively referred to as a perforated circular plate.

The configuration in which a blockage portion having a star hexagonalshape is provided at the central portion of the perforated circularplate shown in FIG. 3 or the circular mesh shown in FIG. 4 ischaracterized as follows:

In the case of a uniform perforated plate, a certain aperture ratio isdefined by the shape of holes (diameter if the holes are circularholes), the distance between adjacent holes (pitch), and the layout ofthe holes. For example, in the case of a general perforated plate inwhich the circular holes have a diameter dk, are spaced at a distance(pitch) P, and are arranged in a 60-degree staggered pattern as shown inFIG. 5, the aperture ratio APR is defined by the following equation:

APR=90.6×dk ² /P ²  (1)

In the case of a mesh material formed by weaving a wire material asshown in FIG. 6, when the aperture width between adjacent wires isrepresented by Am and the diameter of the wire material is representedby dm, the aperture ratio APR is defined by the following equation:

APR=Am ²/(Am+dm)²  (2)

FIGS. 7(a), 7(b), and 7(c) are graphs showing aperture ratios at radialpositions on circular plates.

If there is no blockage portion, the entire surface of the circularplate has a uniform aperture ratio on the average, and thus the apertureratio (APR) is constant at any radial position on the circular plate, asshown in FIG. 7(a).

In the case of a perforated plate having a circular blockage portion atits central part (see FIGS. 25 and 26), the aperture ratio (APR) atrespective diametrical positions becomes zero in a central blockage areahaving a diameter dc and becomes constant in an area whose diameter isgreater than the diameter dc. Thus, the aperture ratio at radialpositions on perforated plate shows the relationship shown in FIG. 7(b).

On the other hand, in the case where a blockage portion having a starhexagonal shape is provided according to the present invention, theaperture ratio APR becomes zero in a central blockage area having adiameter dc, and becomes a certain APR which is calculated as uniformporosity or uniform mesh in an area outside the circumcircle (diameterφdr) of the star hexagon. Therefore, the aperture ratio is graduallyincreased from zero to APR, and the aperture ratio at radial positionsshows the relationship shown in FIG. 7(c).

As described above, the present invention is characterized by theconfiguration in which the aperture ratio is gradually increased towardthe outer circumference of the circular plate.

The aperture ratio represented by the vertical axis of each of thegraphs shown in FIGS. 7(a), 7(b), and 7(c) is an average aperture ratioat each radial position.

If the star polygon is a star hexagon and the angle formed between twohypothetical lines which interconnect two adjacent acute-angle vertexesin an outer circumferential area of the circular plate and the center ofthe perforated circular plate is 60 degrees as shown in FIG. 3, and ifthe holes are arranged in a 60-degree staggered pattern as shown in FIG.5 and the central line of the 60-degree staggered pattern is alignedwith the hypothetical line on the star hexagon, then the holes can berotational symmetry with respect to the center of the perforated plate.In this manner, the holes that are formed and arranged in the rotationalsymmetry are expected to perform a higher action to regulate the flow offluid at the downstream side of the perforated plate so as to beuniformized in the entire chamber than the configuration which is notrotational symmetry.

FIGS. 8(a), 8(b) through 10(a), 10(b) are views showing the flowdistribution of CFD (Computational Fluid Dynamics) in the vicinity ofthe seawater port when the seawater flows into the chamber in the casewhere the flow resistor 23 comprising a perforated circular plate whosecentral portion is blocked in a star hexagonal shape is installedhorizontally. Arrows in figures are as follows: Flow velocity of fluidis shown by a length of arrow and flow direction of fluid is shown by adirection of arrow.

FIGS. 8 (a) and 8(b) are views showing the distribution in the vicinityof the seawater port of the chamber when the seawater flows into thechamber, in the B1-B1 cross-section passing through points ofintersection between the star hexagon and the incircle on the perforatedplate. Specifically, FIG. 8(b) is a plan view of the flow resistor 23having a structure identical to the structure shown in FIG. 3, and FIG.8(a) is a view showing the flow distribution in the vicinity of theseawater port of the chamber, in the B1-B1 cross-section of FIG. 8(b).In FIG. 8(b), the small holes 23 h outside the star hexagon are omittedfrom illustration.

As shown in FIG. 8(a), because the fluid flows into the chamber CH fromthe seawater port P2 having a small diameter, the fluid near the port ofthe chamber has a velocity distribution having a large stream at thecentral part of the chamber. The high-velocity flow of fluid at thecentral part collides with the circular blockage portion, at the centerof the perforated plate, facing the port, and then the flow of fluid isdirected horizontally along the plate toward the outer circumference ofthe chamber. The fluid passes through the perforated plate only from thesmall holes formed at the outer circumferential portion of theperforated plate and flows downstream, and part of the horizontal flowof fluid is directed upwardly along the side surface of the chamber,thus generating large vortices at the outer circumferential portion ofan upstream space that is partitioned by the perforated plate. At thistime, the flow of fluid collides with the blockage portion of theperforated plate and is then directed toward the outer circumference ofthe chamber, and the high-velocity fluid which flows into the chamberfrom the port is slowed down. The flow of fluid which has passed throughthe small holes at the outer circumferential portion of the perforatedplate is directed towards the outer circumferential side once at itscentral part, and then gathers in the central part of the chamber again.This is because the vortex represented by Vx is generated at the backside of the blockage portion of the perforated plate. Further, vortexesVx are also generated in an outer circumferential region at the upstreamside of the evaluation section A-A that is spaced by a predetermineddistance from the perforated plate. Here, the vortexes Vx become complexflows having velocities that include vertical components with respect toa two-dimensional plane of the cross-section shown in FIG. 8(a).

FIGS. 9(a) and 9(b) are views showing the flow distribution of CFD(Computational Fluid Dynamics) in the vicinity of the seawater port ofthe chamber when the seawater flows into the chamber, in the B2-B2cross-section passing through the intermediate between a point ofintersection between the star hexagon and the incircle and a point ofintersection between the star hexagon and the circumcircle on theperforated circular plate. Specifically, FIG. 9(b) is a plan view of theflow resistor 23 having a structure identical to the structure shown inFIG. 3, and FIG. 9(a) is a view showing the flow distribution in thevicinity of the seawater port of the chamber, in the B2-B2 cross-sectionof FIG. 9(b). In FIG. 9(b), the small holes 23 h outside the starhexagon are omitted from illustration.

As shown in FIG. 9(a), vortexes Vx are generated at the back side of theblockage portion of the perforated plate and in the outercircumferential region at the upstream side of the evaluation sectionA-A, as with the results shown in FIG. 8(a). However, the central vortexVx shown in FIG. 9(a) is larger than the central vortex Vx shown in FIG.8(a), and the outer circumferential vortexes Vx shown in FIG. 9(a) aresmaller than the outer circumferential vortexes Vx shown in FIG. 8(a).The main flows in a two-dimensional plane of the cross-section shown inFIG. 9(a) are directed in a downward direction.

FIGS. 10(a) and 10(b) are views showing the flow distribution of CFD(Computational Fluid Dynamics) in the vicinity of the seawater port ofthe chamber when the seawater flows into the chamber, in the B3-B3cross-section passing through points of intersection between the starhexagon and the circumcircle on the perforated plate. Specifically, FIG.10(b) is a plan view of the flow resistor 23 having a structureidentical to the structure shown in FIG. 3, and FIG. 10(a) is a viewshowing the flow distribution in the vicinity of the seawater port ofthe chamber, in the B3-B3 cross-section of FIG. 10(b). In FIG. 10(b),the small holes 23 h outside the star hexagon are omitted fromillustration.

As shown in FIG. 10(a), the central blockage portion has a greaterproportion, and thus flows along the cylindrical wall in the chamberfrom the outer circumferential portion of the chamber are formed. Alarge vortex Vx is formed in the central part of the chamber, andcomplex flows having velocities that include vertical components withrespect to a two-dimensional plane of the cross-section shown in FIG.10(a) are formed. In this case, a state in which a plurality of complexvortexes are mixed in the vortex Vx is developed.

Although the behavior of the flow in the B1-B1 cross-section and thebehavior of the flow in the B3-B3 cross-section are extremes, they cantake place in limited pinpoint cross-sections in each of the threecross-sections. The behavior of the flow intermediate between theextreme behaviors of the flows becomes nearly the same as the behaviorof the flow in the B2-B2 cross-section. As a consequence of thebehaviors of the flows in the three types of cross-sections, the flowsat the back side of the perforated circular plate are less likely tovary, but can be uniformized even if the flow velocity of the fluidflowing in from the port is changed over a wide range.

The flows shown in FIGS. 27 and 28 gather only in the central part ofthe chamber. However, by providing a radial distribution of apertureratios allocated to the blockage portion, the main flows that passthrough the perforated plate are dispersed in a radial direction of thechamber.

Similarly, the fluid which has flowed in from the concentrated seawaterport P1 disposed at the lower part of the chamber collides with theblockage portion at the central part of the perforated plate and is thusslowed down, and thus a uniform flow is formed from the small holes atthe circumferential portion of the perforated plate over the entireplane of the chamber. Therefore, the fluid between the perforated platesflows in and out in a state of a uniform flow in the horizontalcross-section of the chamber, and thus uniform pushing and pulling ofthe seawater and the concentrated seawater are performed in the entirecross-section. By this action, when the seawater and the concentratedseawater are pushed and pulled, mixing of the seawater and theconcentrated seawater having different salt concentrations can besuppressed.

Here, in the energy recovery apparatus according to the presentinvention, the pushing and pulling are switched so that the mixing zoneof the seawater and the concentrated seawater reciprocates between theflow resistors which are respectively disposed at the seawater port P2side and the concentrated seawater port P1 side in the chamber.Therefore, the mixing zone of the seawater and the concentrated seawateris present in the portion represented by La between the flow resistors23, 23 in FIG. 2. The seawater flowing in from the seawater port P2provided at the upper part of the chamber becomes a uniform flow by theflow resistor 23 in the horizontal cross-section of the chamber at thedownstream side of the flow resistor 23, but this flow is changed alsoby flow resistance of fluid flowing out from the concentrated seawaterport P1 side as a discharge side. Specifically, this flow is changedalso by the combination with the flow resistor 23 disposed at theconcentrated seawater port side. Therefore, the Computational FluidDynamics at the time of inflow shown in FIGS. 8(a), (b) through 10(a),(b) takes into account the resistance of the flow resistor 23 at thedischarge side.

In this manner, the uniformizing action of flow by the flow resistor atthe inflow side in the present invention varies depending on thearrangement of the flow resistor and the port at the discharge side.Because the energy recovery apparatus repeats inflow and discharge ofthe seawater and the concentrated seawater alternately, in addition touniformization of flow in one direction, the flow of discharge when thefluid flows in the opposite direction should be considered.

FIG. 11(a) is a schematic cross-sectional view showing an energyrecovery chamber of an enemy recovery apparatus according to anotherembodiment of the present invention. As shown in FIG. 11(a), in thechamber, a first perforated plate 24 is provided horizontally at apositon spaced by a distance L1 from the seawater port P2, and similarlya first perforated plate 24 is provided horizontally at a positionspaced by a distance L1 from the concentrated seawater port P1. Further,second perforated plates 25 are provided horizontally at positionsspaced by a distance L2 from the respective first perforated plates 24.The first perforated plate 24 and the second perforated plate 25constitute a flow resistor 23.

FIG. 11(b) is a plan view showing the respective perforated platesdisposed in the energy recovery chamber shown in FIG. 11(a). FIG. 11(b)shows the first perforated plate 24 and the second perforated plate 25at the seawater port side and the second perforated plate 25 and thefirst perforated plate 24 at the concentrated seawater port side fromthe top to the bottom. The first perforated plate 24 constituting theflow resistor disposed in the energy recovery chamber shown in FIG.11(a) comprises a single perforated plate which has a plurality of smallholes formed outside a star hexagon having a central hypothetical circleas an incircle and an outer hypothetical circle as a circumcircle andhas no hole inside the star hexagon (central side). The first perforatedplate 24 has the same structure as that in FIG. 3. The first perforatedplate 24 may comprise a porous plate which has a central blockageportion and an outer circumferential portion comprising a mesh materialas shown in FIG. 4. Further, the second perforated plate 25 comprises acircular plate having small holes formed at regular intervals over theentire surface thereof. The second perforated plate 25 may comprise acircular plate made of a mesh material.

FIG. 12 is an enlarged plan view of the second perforated plate 25 shownin FIG. 11(b). As shown in FIG. 12, the second perforated plate 25comprises a circular plate having an outer diameter of φD equal to aninner diameter of the chamber, and small holes 25 h having a diameterφdk2 are formed at regular intervals over the entire surface of thecircular plate as shown in FIG. 5.

FIGS. 13(a), 13(b) through FIGS. 15(a), 15(b) are views showing the flowdistribution in the vicinity of the seawater port when the seawaterflows into the chamber in the case where the perforated circular plates24 whose central portions are blocked in a star hexagonal shape as shownin FIGS. 11(a), 11(b) are disposed as first perforated plates atrespective positions spaced by a distance L1 from the seawater port andthe concentrated seawater port, and the second perforated plates 25having uniform small holes over their entire surfaces are disposed atrespective positions spaced by a distance L2 from the first perforatedplates 24, the first and second perforated plates 24, 25 constituting aflow resistor 23 being disposed horizontally in the chamber. Arrows infigures are as follows: Flow velocity of fluid is shown by a length ofarrow and flow direction of fluid is shown by a direction of arrow

FIGS. 13(a) and 13(b) are views showing the flow distribution in thevicinity of the seawater port of the chamber when the seawater flowsinto the chamber, in the B1-B1 cross-section passing through points ofintersection between the star hexagon and the incircle on the perforatedplate. Specifically, FIG. 13(b) is a plan view of the first perforatedplate 24 having a structure identical to the structure shown in FIG. 3,and FIG. 13(a) is a view showing the flow distribution in the vicinityof the seawater port of the chamber, in the B1-B1 cross-section of FIG.13(b). In FIG. 13(b), the small holes 23 h outside the star hexagon areomitted from illustration.

FIGS. 14(a) and 14(b) are views showing the flow distribution in thevicinity of the seawater port of the chamber when the seawater flowsinto the chamber, in the B2-B2 cross-section passing through theintermediate between a point of intersection between the star hexagonand the incircle and a point of intersection between the star hexagonand the circumcircle on the perforated plate. Specifically, FIG. 14(b)is a plan view of the first perforated plate 24 having a structureidentical to the structure shown in FIG. 3, and FIG. 14(a) is a viewshowing the flow distribution in the vicinity of the seawater port ofthe chamber, in the B2-B2 cross-section of FIG. 14(b). In FIG. 14(b),the small holes 23 h outside the star hexagon are omitted fromillustration.

FIGS. 15(a) and 15(b) are views showing the flow distribution in thevicinity of the seawater port of the chamber when the seawater flowsinto the chamber, in the B3-B3 cross-section passing through points ofintersection between the star hexagon and the circumcircle on theperforated plate. Specifically, FIG. 15(b) is a plan view of the firstperforated plate 24 having a structure identical to the structure shownin FIG. 3, and FIG. 15(a) is a view showing the flow distribution in thevicinity of the seawater port of the chamber, in the B3-B3 cross-sectionof FIG. 15(b). In FIG. 15(b), the small holes 23 h outside the starhexagon are omitted from illustration.

The state of the flows at the downstream side of the first perforatedplate shown in FIGS. 13(a), 13(b) through FIGS. 15(a), 15(b) can bedescribed in substantially the same as the state of the flows describedabove with reference to FIGS 8(a), 8(b) through 10(a), 10(b). Byproviding the first perforated plate 24 and the second perforated plate25 spaced by a distance L2 from the first perforated plate 24, the flowof fluid that flows into the chamber at a high velocity from the porthaving a small diameter is dispersed over the entire plane of thechamber section by the first perforated plate 24, and the flow whosevelocity distribution has been uniformized by the first perforated plate24 passes through the second perforated plate 25 having the small holesformed over its entire surface to allow the flow of fluid at thedownstream side of the second perforated plate 25 to be regulated into amore uniform flow. Therefore, the flow of fluid becomes closer to theflow whose velocities and directions are the same in the A-Across-section spaced by a certain distance from the second perforatedplate 25 to the chamber center, thus achieving a more uniform flow.

The first perforated plate has a function to cause the high-velocityflow of fluid from the seawater port (or the concentrated seawater port)to collides with the central circular blockage portion, thereby reducingthe velocity of the flow and dispersing the flow toward the outercircumferential part of the first perforated plate. Then, the firstperforated plate disperses the flow of fluid to make the flow at thedownstream side of the perforated plate a “uniform” velocitydistribution defined in FIG. 29 in the circular cross-section of thechamber by the hole passages whose aperture ratios are graduallyincreased from the center toward the outer circumference of the firstperforated plate. The second perforated plate has a function to furtheruniformize velocity differences in the velocity distribution which stillremain in the flow dispersed by the first perforated plate.

Even if the uniformity of the flows that have passed through the firstperforated plate is lost by the inflow velocity, by adding a function touniformize the velocity distribution by the second perforated plate, theflow resistor can cope with a wider range of inflow velocities. Thismeans that the energy recovery apparatus can cope with a wide range offlow rates to be processed.

FIG. 16 is a plan view showing another flow resistor. In the case wherethe flow resistor comprises a single perforated plate shown in FIG. 2,the flow resistor 23 shown in FIG. 16 is used in place of the flowresistor 23 shown in FIG. 3. In the case where the flow resistorcomprises two perforated plates shown in FIGS. 11(a) and 11(b), the flowresistor 24 shown in FIG. 16 is used in place of the flow resistor 24shown in FIGS. 11(a) and 11(b). As shown in FIG. 16, the flow resistor23 is in the shape of a circular plate having an outer diameter (φD)equal to the inner diameter of the chamber. The flow resistor 23comprises a single perforated plate which has a plurality of small holeshaving a diameter φdk1 formed outside a star tetragon having a centralhypothetical circle (φdc) as an incircle and an outer hypotheticalcircle (φdr) as a circumcircle and has no hole inside the star tetragon(central side). Specifically, the flow resistor 23 comprises aperforated plate which is blocked in its central portion and in part ofits outer circumferential portion.

The diameter (φdc) of the central hypothetical circle on the perforatedplate is equal to or slightly greater than the inner diameter φds of theseawater port and the inner diameter φdb of the concentrated seawaterport in FIG. 2, so that the high-velocity flow of fluid flowing in fromeach of the ports collides with the blockage portion and is slowed down.However, if the blockage portion is excessively larger than each of theports, the flows of fluid passing through a plurality of small holes 23h provided at an outer circumferential side of the perforated plate arelocalized in an outer circumferential region, thus making the flowuniformizing action small adversely. Therefore, the diameter of thecentral hypothetical circle is substantially the same as the innerdiameter of each of the ports.

The diameter (φdr) of the hypothetical circle that circumscribes thestar tetragon is smaller than the outer diameter (φD) of the flowresistor 23.

FIG. 17 is an enlarged plan view showing the layout of holes formed inthe perforated plate shown in FIG. 16. Holes having a diameter φdk aredisposed on orthogonal axes and spaced from each other by a distance(pitch) P. The layout of the holes is referred to as a parallel layout,and the aperture ratio APR of the holes is calculated according to thefollowing equation:

APR=78.5×dk ² /P ²   (3)

The central blockage portion of the circular plate is in a shape of astar tetragon and the holes formed in the circular plate are disposed ina parallel layout, and thus the chamber is configured to be 90-degreerotationally symmetric.

The flow resistor comprising the perforated circular plate shown in FIG.16 is also characterized in that the aperture ratio thereof is graduallyincreased from the outer diameter of the central hypothetical circletoward the outer diameter of the perforated circular plate. Theperforated circular plate shown in FIG. 16 which includes the centralblockage portion of the star tetragon is different in the aperture ratioand the gradient of a gradual increase of the aperture ratio from theperforated circular plate shown in FIG. 2 which includes the centralblockage portion of the star hexagon.

The flow resistor 23 comprising the perforated plate which is blocked bythe star tetragon, has a function to regulate the flow of fluid at thedownstream side of the flow resistor 23 so as to be uniformized in theentire chamber by imparting an appropriate flow resistance to the flowof fluid at the upstream side of the flow resistor 23 in the chamber CH.Both the star tetragon (FIG. 16) and the star hexagon (FIG. 2) have anexcellent flow uniformizing action, and indicate that the configurationin which the aperture ratio is gradually increased from the outerdiameter of the central hypothetical circle toward the outer diameter ofthe perforated circular plate is effective to uniformize the flow offluid.

FIG. 18 is a cross-sectional view showing a chamber of the energyrecovery apparatus according to still another embodiment of the presentinvention. The chamber according to the present embodiment has aconfiguration in which the upper seawater port is divided into two portscomprising a seawater inflow port P2 _(IN) and a seawater discharge portP2 _(out), and the seawater inflow port P2 _(IN) and the seawaterdischarge port P2 _(OUT) are arranged at positions spaced radially fromthe central axis of the chamber. Further, a holed circular plate 31having a hole at a central part thereof is disposed at a position spacedby a distance Lp from the ports P2 _(IN), P2 _(OUT). A first perforatedplate 24 having a central blockage portion is provided at a positionspaced by a distance L1 from the holed circular plate 31, and a secondperforated plate 25 having holes formed at regular intervals over theentire surface thereof is provided at a position spaced by a distance L2from the first perforated plate 24.

FIG. 19 is a plan view of the holed circular plate 31. The holedcircular plate 31 has an outer diameter equal to the inner diameter (φD)of the chamber and has a circular hole having a diameter (φdp) at acentral part thereof. By providing the holed circular plate 31, thefluid which has flowed in from the port is regulated so that the fluiddoes not flow through the outer circumferential portion of the holedcircular plate 31, but flows through the hole having a diameter φdp atthe central part of the holed circular plate 31 toward the flow resistor23. Therefore, even if the port is not disposed at the central part ofthe chamber, the flow of fluid can be changed once into the flow throughthe central part of the chamber, and then this flow can be diffuseduniformly in an outer circumferential direction and regulated by theflow resistor 23 at the downstream side. Therefore, a uniform flow isformed in the cylindrical chamber.

When the high-velocity seawater flows into the chamber CH from theseawater inflow port P2 _(IN) disposed at an eccentric position from thechamber axis, the flow of the seawater collides with the plate portionhaving no hole at the circumferential portion of the holed circularplate 31 and is then dispersed in the space partitioned by the holedcircular plate 31 and located at the seawater port side. Then, theseawater passes through the hole formed at the central portion of theholed circular plate 31 and flows at a high velocity toward the centralportion of the first perforated plate 24. Thereafter, the flow of fluidcollides with the blockage portion having no hole at the central portionof the first perforated plate 24, and is then dispersed toward the outercircumference of the chamber and slowed down. The flow of fluid at thedownstream side of the first perforated plate 24 is the same as the flowdescribed and shown in FIGS. 8(a), (b) to 10(a), (b).

The holed circular plate 31 has the same effect as that of thearrangement in which the seawater inflow port P2 _(IN) disposed at aneccentric position from the chamber axis is disposed so as to be alignedwith the chamber axis in the chamber.

FIG. 20 is a cross-sectional view of the chamber in the energy recoveryapparatus according to still another embodiment of the presentinvention.

The configuration at the seawater port side of the chamber in FIG. 20 isthe same as that in the embodiment shown in FIG. 18. However, thechamber of the present embodiment is different in that the concentratedseawater port at the lower part of the chamber is formed at the sidesurface of the chamber. Specifically, since the concentrated seawaterport P1 is formed at the side surface of the chamber, the concentratedseawater is supplied and discharged in a direction perpendicular to theaxial direction of the chamber (radial direction of the chamber).Further, the holed circular plate 31 having a hole formed at the centralpart thereof is provided at a position spaced by a distance Lp from thechamber end surface at the concentrated seawater port side, and thefirst perforated plate 24 is provided at a position spaced by a distanceL1 from the holed circular plate 31. Furthermore, the second perforatedplate 25 is provided at a position spaced by a distance L2 from thefirst perforated plate 24.

The holed circular plate 31 has the same configuration as that shown inFIG. 19, the first perforated plate 24 has the same configuration asthat shown in FIG. 3 or FIG. 4, and the second perforated plate 25 hasthe same configuration as that shown in FIG. 12.

In FIG. 20, the fluid which has flowed in from the concentrated seawaterport P1 at the chamber side surface is regulated so that the fluid flowsthrough the hole having a diameter (φdp) at the central portion of theholed circular plate 31 toward the flow resistor 23. Therefore, even ifthe port is disposed at the side surface of the chamber, the flow offluid can be changed once into the flow through the central part of thechamber, and then this flow can be diffused uniformly in an outercircumferential direction and regulated by the flow resistor 23 at thedownstream side. Therefore, a uniform flow is formed in the cylindricalchamber.

When the high-velocity concentrated seawater flows into the chamber CHin a direction perpendicular to the chamber axis from the concentratedseawater port P1 disposed at the side surface of the chamber, in thespace partitioned by the holed circular plate 31 and located at theconcentrated seawater port side, part of the concentrated seawater flowsout through the hole formed at the central portion of the holed circularplate 31, and part of the concentrated seawater forms vortices in thespace and spreads in the space. Then, the concentrated seawater flowsout through the hole formed at the central portion of the holed circularplate 31. Thereafter, the concentrated seawater flows at a high velocitytoward the central portion of the first perforated plate 24 from theholed circular plate 31, and the flow of the concentrated seawatercollides with the blockage portion having no hole at the central portionof the first perforated plate 24, and is then dispersed toward the outercircumference of the chamber and is slowed down. The flow of fluid inthe downstream side after flowing in through the first perforated plate24 becomes the upside-down flow, which has been described and shown inFIGS. 8(a), (b) to 10(a), (b).

As described above, the uniformizing action of the flow by the flowresistor at the inflow side varies also depending on the arrangement ofthe flow resistor 23 and the port at the discharge side. By providingthe holed circular plate 31 having a hole at the central part thereof,the inflow position of the fluid to the flow resistor 23 becomes acenter of the chamber, regardless of the arrangement of the port. As inthe embodiments shown in FIG. 18 and FIG. 20, even if the actualseawater port and the concentrated seawater port are not locatedcentrally, the hole formed at the central part of the holed circularplate 31 disposed between each port and the flow resistor 23 can beconsidered as a hypothetical seawater port or concentrated seawater portin the chamber. Thus, the operation and the effect equivalent to theinvention according to the embodiments shown in FIG. 2 and FIGS. 11(a),(b) can be obtained.

In this manner, in order to form a uniform flow in the chamber space(the portion of La in FIGS. 2, 11(a), 18 and 20) in which the seawaterand the concentrated seawater are pushed and pulled, the presentinvention has a configuration which has the inflow and discharge port(or hole) at the chamber center position, the flow resistor, the chamberspace in which the seawater and the concentrated seawater are pushed andpulled, the flow resistor, and the inflow and discharge port (or hole)at the chamber center position. Thus, even if the fluid flows in theopposite direction, the same configuration is formed and the samesequence of flow is formed. In this manner, in the inflow and dischargeof the fluid, the flow resistance arranged in the chamber has symmetry.

The flow resistors arranged in the chamber between the ports arerotationally symmetric about a chamber central axis, and the flowresistance of inflow and discharge of fluid in the radial direction ofthe chamber is arranged to be rotationally symmetric. As in theembodiment in FIG. 18, in the case where one of the ports is not locatedat the center of the chamber, the internal structure of the chamberbetween the hole at the central portion of the holed circular plate 31and the central port is rotationally symmetric about the chamber centralaxis. As in the embodiment shown in FIG. 20, in the case where bothports are not located at the chamber center, the internal structure ofthe chamber between the holes at the central portions of both the holedcircular plates 31 is rotationally symmetric about the chamber centralaxis.

When the discharge of the concentrated seawater in the case where theholed circular plate 31 is not provided at the concentrated seawaterside in FIG. 20 is considered, the flow of the concentrated seawater isoffset in a radial direction because the concentrated seawater is easilydischarged from the flow resistor 23 at the concentrated seawater portside to the port P1 side at the left side located downstream of the flowresistor 23. As a result, considering the seawater inflow, the action bythe flow resistor 23 at the seawater side is affected by non-uniformityof the flow resistance downstream of the flow resistor 23, and thus theuniformizing action is lost. This is because in the case where the holedcircular plate 31 in the embodiment of FIG. 20 is not provided, therotational symmetry about the chamber central axis in the arrangementbetween the ports is lost and the characteristic of the structuralsymmetry of the present invention is lost. Thus, according to thepresent invention, the action of the flow resistors arranged in thechamber between the ports (holes) is made to be rotationally symmetricabout the chamber central axis, and thus the flow resistance in a radialdirection of the chamber becomes rotationally symmetric, thus forming auniform flow in the pushing and pulling space between the flowresistors.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made without departing from the scopeof the appended claims.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a seawater desalination systemfor desalinating seawater by removing salinity from the seawater and anenergy recovery apparatus which is preferably used in the seawaterdesalination system.

REFERENCE SIGNS LIST

1 seawater supply line

2 high-pressure pump

3 discharge line

4 reverse-osmosis membrane-separation apparatus

5 concentrated seawater line

6 control valve

7, 9 valve

8 booster pump

10, 20 energy recovery chamber

11 energy recovery apparatus

12, 22 end plates

13, 23 flow resister

14, 24 first perforated plate

15, 25 second perforated plate

16 piston

17 concentrated seawater discharge line

21 chamber body

23 h hole

30 circular plate

31 holed circular plate

CH chamber

P1 concentrated seawater port

P2 seawater port

P2 _(IN) seawater inflow port

P2 _(OUT) seawater discharge port

1. An energy recovery apparatus for converting pressure energy ofconcentrated seawater discharged from a reverse-osmosismembrane-separation apparatus to pressure energy of seawater in aseawater desalination system for producing fresh water from the seawaterby supplying the seawater pressurized by a pump to the reverse-osmosismembrane-separation apparatus to separate the seawater into fresh waterand concentrated seawater, the energy recovery apparatus comprising: acylindrical chamber having a space for containing concentrated seawaterand seawater therein, the chamber being installed such that alongitudinal direction of the chamber is placed in a vertical direction;a concentrated seawater port provided at a lower part of the chamber forsupplying and discharging the concentrated seawater; a seawater portprovided at an upper part of the chamber for supplying and dischargingthe seawater; a flow resistor provided at a concentrated seawater portside in the chamber; and a flow resistor provided at a seawater portside in the chamber; wherein each of the flow resistor provided at theconcentrated seawater port side and the seawater port side comprises atleast one performed circular plate; and wherein each perforated circularplate has a plurality of holes formed in an outer circumferential areaoutside a hypothetical circle which is concentric with the perforatedcircular plate and has a predetermined diameter, the holes being formedso that an aperture ratio is varied in a circumferential direction ofthe perforated circular plate in the outer circumferential area outsidethe hypothetical circle having the predetermined diameter.
 2. The energyrecovery apparatus according to claim 1, wherein an area of theperforated circular plate that is free of the holes is an area of apolygon which has a circle having a predetermined diameter as anincircle and a circle having a diameter equal to or smaller than theouter diameter of the perforated circular plate and greater than thediameter of the hypothetical circle as a circumcircle.
 3. The energyrecovery apparatus according to claim 1, wherein the flow resistor isrotationally symmetric about a central axis of the cylindrical chamber.4. The energy recovery apparatus according to claim 1, wherein theperforated circular plate serves as a first perforated plate, and asecond perforated plate is provided to be spaced by a predetermineddistance from the first perforated plate.
 5. The energy recoveryapparatus according to claim 1, further comprising a doughnut-shapedcircular plate having an opening at a center thereof provided betweenone of the concentrated seawater port and the seawater port or both ofthe concentrated seawater port and the seawater port, and the flowresistor.
 6. A seawater desalination system for producing fresh waterfrom seawater by supplying the seawater pressurized by a pump to areverse-osmosis membrane-separation apparatus to separate the seawaterinto fresh water and concentrated seawater, the seawater desalinationsystem comprising: an energy recovery apparatus according to claim 1 forconverting pressure energy of the concentrated seawater discharged fromthe reverse-osmosis membrane-separation apparatus to pressure energy ofthe seawater.